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ISSN: 1689-832X
Journal of Contemporary Brachytherapy
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5/2024
vol. 16
 
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abstract:
Original paper

Sensitivity of contributing factors to heterogeneity corrections in dosimetry of ocular brachytherapy using iodine-125 COMS plaques

Yongsook C. Lee
1, 2
,
Mehran Nik Akhtar
3
,
Yongbok Kim
4
,
Jae Won Jung
5

  1. Department of Radiation Oncology, Miami Cancer Institute, Baptist Health South Florida, Miami, Florida, USA,
  2. Department of Radiation Oncology, Herbert Wertheim College of Medicine, Florida International University, Miami, Florida, USA
  3. West Physics, Atlanta, Georgia, USA
  4. Department of Radiation Oncology, Duke University, Durham, North Carolina, USA
  5. Department of Radiation Oncology, East Carolina University, Greenville, North Carolina, USA
J Contemp Brachytherapy 2024; 16, 5: 371–382
Online publish date: 2024/10/18
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Purpose:
To investigate sensitivity of contributing factors to heterogeneity corrections in ocular brachytherapy using iodine-125 (125I) Collaborative Ocular Melanoma Study (COMS) plaques.

Material and methods:
Using egs_brachy, Monte Carlo (MC) simulations were performed for 125I COMS plaques (model: IsoAid IAI-125A). Homogeneous dose (DHOMO) was estimated under the American Association of Physicists in Medicine Task Group-43 assumptions, with 85 Gy prescribed to a 5 mm depth. Heterogeneous doses (DHETERO#1-#7) were determined with medium-heterogeneity for the following scenarios: #1. Modulay backing (M) alone, #2. Silastic insert (S) alone, #3. Interseed attenuation effects (I) alone, #4. M + S, #5. M + I, #6. S + I, and #7. M + S + I. For scenarios #1-#7, heterogeneity correction factors (HCFs) were derived from DHETERO#1-#7/DHOMO for central axis points up to 22 mm, and for off-axis points (i.e., optic disc, macula, and lens) as a function of distance from tumor to optic disc (DT) or macula (MT), and tumor dimensions in the direction of optic disc (BD) or macula (BM).

Results:
For a 16-mm diameter COMS plaque, in central axis, the dominant factor to heterogeneity corrections (dose reduction by 9.5-13.8% with heterogeneity) was Silastic insert at a depth ≤ 5 mm, whereas both Modulay backing and Silastic insert were contributing (range, 12.2-19.0%) at deeper depths. For off-axis, Silastic insert was the major factor to heterogeneity corrections (≥ 11.8%) at DT (MT) ≤ 6.9 (7.6) mm for optic disc (macula) and (range, 8.9-10.0%) at MT > 18.5 mm for lens, but both Modulay backing and Silastic insert were contributing (range, 18.3-24.4%) at farther DT (MT) for optic disc (macula) and (range, 12.8-18.2%) at MT ≤ 18.5 mm for lens. Interseed effects were small (≤ 1.7% for central axis and ≤ 2.3% for off-axis). Data for the other six plaques (10-14 mm and 18-22 mm diameter) presented similar trends.

Conclusions:
The use of universal HCF(s) for the two dominant factors determined in this study will facilitate heterogeneous dose estimates for each clinical scenario without an MC tool.

keywords:

COMS plaques, heterogeneity corrections, 125I, Monte Carlo simulations, ocular brachytherapy, sensitivity of contribution factors

 
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